The invention relates to a pump for a high-pressure cleaning apparatus for delivering a cleaning liquid, said pump comprising at least one pump chamber into which at least one reciprocally movable piston plunges and which is connected via at least one inlet valve to a suction conduit and is connected via at least one outlet valve to a pressure conduit, and a bypass conduit leading from the pressure conduit to the suction conduit and having arranged therein a bypass valve, the valve body of which is connected, via a piston rod, to a control member which displaces the valve body as a function of the flow rate of the cleaning liquid in the pressure conduit to a closed position or an open position and displaces a switch plunger which is coupled to the valve body to a first or a second switch position to actuate a switch element.
Pumps of this type are known from DE 196 07 881 A1. They can be used to pressurize and then direct a cleaning liquid, for example water, towards an object, using for example a pressure hose which can be connected to the pressure conduit, and a nozzle head which is arranged at the free end of the pressure hose. In order for the mechanical loading of the pump as well as heat losses to be reduced, the cleaning liquid pumped by the pump when the nozzle head is closed is recirculated against as low a resistance to flow as possible, i.e. it is recycled from the pressure conduit back to the suction conduit so that the pressure in the pressure conduit can be reduced. For this purpose, the pressure conduit is connected to the suction conduit via a bypass conduit, said bypass conduit having a bypass valve arranged therein. In working mode of the pump, i.e. when the nozzle head is open, the bypass valve closes the flow communication between the pressure conduit and the suction conduit. If the nozzle head is closed, the bypass valve opens the flow communication between the pressure conduit and the suction conduit, thereby reducing the pressure existing in the pressure conduit. For this purpose, the valve body of the bypass valve is connected via a piston rod to a control member that displaces the valve body as a function of the flow rate of the cleaning liquid in the pressure conduit to a closed position or an open position. The flow rate of the cleaning liquid in the pressure conduit is a function of whether the nozzle head is open or closed. If the nozzle head is closed, the flow rate decreases, thus causing the control member to displace the valve body of the bypass valve to its open position so as to allow the cleaning liquid under pressure to flow to the suction conduit against as low a resistance to flow as possible. If the nozzle head is opened, the flow rate in the pressure conduit increases, thus causing the control member to displace the valve body of the bypass valve to the closed position so that the pump transitions to normal working mode.
In addition to its function of controlling the bypass valve, the control member provides the function of displacing a switch plunger that is coupled to the valve body. The switch plunger can actuate a switch element. The switch element may, for example, turn on and off a drive device of the pump, preferably an electric motor. Actuating the switch plunger thus allows the pump to be activated and deactivated. If the flow of liquid in the pressure conduit is stopped, then on the one hand the bypass valve opens the flow communication between the pressure conduit and the suction conduit so that the pressure existing in the pressure conduit can be reduced, and on the other hand the switch plunger is displaced to a first switch position so that the switch element turns off the pump. However, for the switch plunger to reach said switch position, it has to overcome a certain travel distance. To this end, the valve body should, where practicable, be acted upon by as great a differential pressure as possible in order to assist the movement of the switch plunger coupled thereto. However, as a result of such a differential pressure, the cleaning liquid in the bypass conduit suffers significant losses of flow. This leads to heat losses in and to considerable mechanical load on the pump. Provided that it is ensured that, after the flow communication between the pressure conduit and the suction conduit has been opened, the pump is reliably switched off, short-term loading of the pump in recirculation mode can be tolerated. However, should the pump fail to be turned off, due for instance to a defect in the switch element, then the pump's continued running in recirculation mode at a persistently high differential pressure at the valve body of the bypass valve may result in damage thereto.
Therefore, it is an object of the present invention to improve a pump of the type mentioned at the outset such that, when the pump transitions to recirculation mode, the switch plunger reliably overcomes a certain travel distance to actuate the switch element, while the cleaning liquid suffers as low losses of flow as possible when the pump remains in recirculation mode for extended periods of time.